ABSTRACT: Nitrification is a key microbial process in the nitrogen cycle of the ocean that largely determines the pool sizes of nitrite, nitrate and ammonium but hardly affects the total pool of inorganic nitrogen (except through N₂O losses to the atmosphere). This study explored the regulation of nitrification and its dependency on particle and ammonium concentrations. Incubation experiments were conducted in which particles of different origin were added to Baltic seawater, followed by ¹⁵NH₄⁺-labelling and droplet digital PCR. Both nitrification rates according to particle size (>3 µm and 0.22-3 µm) and archaeal and betaproteobacterial ammonia monooxygenase α-subunit gene (amoA) abundances were determined in order to assess the impact of high particle concentrations found in coastal areas of the Baltic Sea. The data show that the addition of particulate organic matter enhanced the nitrification rates in bulk (~805 nmol l^-1 d^-1) but not in the <3 µm fraction (<100 nmol l^-1 d^-1), indicative of a strong influence of the added particles on nitrification rates. The regulation of nitrification rates by the substrate ammonium was suggested by the inverse correlation between these 2 variables. Measurements of amoA gene abundances showed the dominance of ammonium-oxidising bacteria over ammonium-oxidising archaea during most of the 41 d experiment. Abundances of amoA were highest in the particle-associated fraction of the bacteria (≤6.9 × 10⁶ amoA copies l^-1). These findings demonstrate the importance of particle-associated ammonium-oxidising bacteria in the nitrification processes of coastal and estuarine waters containing high particle densities.

中文翻译：

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添加有机颗粒对波罗的海海水中硝化速率和铵氧化剂丰度的影响

摘要：硝化作用是海洋氮循环中的一个关键微生物过程，它在很大程度上决定了亚硝酸盐、硝酸盐和铵的池大小，但几乎不影响无机氮的总池（除了通过 N ₂ O 向大气流失）。本研究探讨了硝化作用的调节及其对颗粒和铵浓度的依赖性。进行了孵化实验，其中将不同来源的颗粒添加到波罗的海海水中，然后进行¹⁵ NH ₄ ⁺标记和液滴数字 PCR。根据粒径（>3 µm 和 0.22-3 µm）和古细菌和β-变形菌氨单加氧酶α-亚基基因（amoA) 丰度是为了评估在波罗的海沿岸地区发现的高颗粒浓度的影响。数据显示，颗粒有机物的添加提高了整体硝化速率 (~805 nmol l ^-1 d ^-1 )，但未提高 <3 µm 部分 (<100 nmol l ^-1 d ^-1 )，表明添加的颗粒对硝化速率的影响很大。这两个变量之间的负相关表明底物铵对硝化速率的调节。amoA基因丰度的测量表明，在 41 d 实验的大部分时间里，铵氧化细菌在铵氧化古细菌中占优势。丰富的amoA在细菌的颗粒相关部分中最高（≤6.9 × 10 ⁶ amoA拷贝 l ^-1）。这些发现证明了与颗粒相关的铵氧化细菌在含有高颗粒密度的沿海和河口水域的硝化过程中的重要性。